The invention relates to an electrically actuatable parking brake system for a pneumatic brake system, having a control valve device which has a control piston, it being possible for the parking brake system to engage at least two operating states, namely a parking state or a driving state, it also being possible for the operating states to be engaged selectively depending on a position of the control piston, and the parking state being present if the control piston is forced into an end position by the force of a spring which is arranged in a spring chamber (space).
The present disclosure is concerned with electrically actuatable parking brake systems as are described, for example, in DE 10 2008 007 877 B3. A central safety-relevant requirement made of systems of this type consists in the fact that a power failure must not lead to a change in the state of the parking brakes; if the parking brake is in its parking state, the parking state has to be maintained even in the case of a power failure, in order to thus prevent the commercial vehicle in every case from rolling away unintentionally; if the parking brake is in a driving state, the parking brake must not be engaged suddenly in the case of a power failure, since this can lead to dangerous situations during driving.
In order to meet these safety-relevant requirements, bistable control valves can be used. The latter can be electrically or pneumatically actuatable. The present invention is concerned with pneumatically actuatable switching valve devices which are incorporated into an electrically actuatable parking brake system such that there is bistability and, therefore, the above-mentioned safety-relevant requirements are met.
Electrically actuatable parking brake systems and, in particular, the pneumatic control valve devices which are mentioned are to be designed such that absolute switching security is ensured. If the pressure conditions in the parking brake system are therefore changed over, in order to bring about a switchover of the control valve device, the control valve device has to be capable of switching reliably, in the case of any external circumstances, in particular even at low temperatures which can impede a switchover of the switching valve device, in particular on account of increased frictional forces.
The invention is based on the object of providing a concept, in which the bistability of a pneumatic control valve device is ensured for an electrically actuatable parking brake system; this is intended to ensure a stable parking state, in particular.
This and other objects are achieved by an electrically actuatable parking brake system for a pneumatic brake system, having a control valve device which has a control piston. It is possible for the parking brake system to engage at least two operating states, namely a parking state or a driving state. It is also possible for the operating states to be engaged selectively depending on a position of the control piston, wherein the parking state is present if the control piston is forced into an end position by the force of a spring which is arranged in a spring chamber. Leakage air, which escapes out of a ventilation connection of the control valve device, is returnable into the spring chamber.
The invention improves upon the generic prior art mentioned at the outset, by virtue of the fact that leakage air which escapes out of a ventilation connection of the control valve device can be returned into the spring chamber. The leakage air which can be returned into the spring chamber can assist the action of the spring, that is to say contribute to its force, by way of which the piston is forced into a position which ensures the parking state.
It can be provided, in particular, that, in addition to the ventilation connection, the control valve device has a first working connection, a second working connection and a pneumatic control input. The control valve device engages the driving state by way of ventilation of the control input, in which driving state the working connections are connected to one another, and in which driving state the second working connection is sealed with respect to the ventilation connection. The control valve device engages a parking state by way of ventilation of the control input and spring force, in which parking state the working connections are sealed with respect to one another and the second working connection is connected to the ventilation connection. In the parking state, the second working connection and the components which are connected downstream of it are to be connected to the ventilation connection, with the result that the system is pressureless. Here, the first working connection which is connected to the compressed air source is to be sealed, in particular, therefore, it is not to be connected to the ventilation connection. Nevertheless, a certain amount of leakage is not ruled out, with the result that leakage air can be guided to the ventilation connection. This leakage air is then fed to the spring chamber in order to assist the maintenance of the parking state.
In this context, it is particularly useful that the second working connection can be coupled to a compressed air consumer, that the pneumatic control input is coupled at least indirectly to the control input of a relay valve for the parking brake, that the ventilation connection is coupled to a control and ventilation valve device, and that a ventilation output of the control and ventilation valve device is coupled to the spring chamber. The control and ventilation valve device, which is preferably designed as a solenoid valve, supplies the pneumatic signals for the control valve device. In particular, it also provides a ventilation output, via which the system pressure of the parking brake system can be dissipated, whereby at the same time a control signal is provided for the control valve device. The leakage air which escapes via the ventilation connection of the control and ventilation valve device can then be utilized, in order to be fed to the spring space of the control valve device.
The invention is developed in a particularly advantageous manner by virtue of the fact that the ventilation output of the control and ventilation valve device is coupled to a component which generates a throttling or accumulating effect, and a leakage air return line which leads to the spring chamber branches off between the ventilation output and the component. The component which generates the throttling or accumulating effect ensures that the leakage air does not simply flow out to atmosphere, but rather can be recirculated, in order to assist the spring action in the spring chamber of the control and valve device.
Here, it can be provided, in particular, that the component which generates the throttling or accumulating effect is a non-return valve.
It is then particularly useful that the opening and closing pressures of the non-return valve are selected in such a way that the non-return valve opens at a first pressure level below a system pressure, in order to permit ventilation of the parking brake system, and closes at least partially at a second pressure level below the first pressure level, in order to generate a throttling or accumulating effect. The non-return valve therefore does not impede, or only impedes to a noncritical extent, the ventilation of the system during the transition from the driving state into the parking state. Nevertheless, the throttling or accumulating effect of the non-return valve is sufficient to at least partially prevent the leakage air from flowing out to atmosphere in the parking state and to build up a pressure in the spring chamber of the control valve device.
In this context, it can be particularly useful that the opening and/or closing pressure of the non-return valve can be adjusted. This can be advantageous, since the leakage rate of the control valve device changes during the system service life, in particular can increase. An adaptation can be carried out here by changing of the switching pressures of the non-return valve, in particular in the context of maintenance, in order to ensure reliable functioning of the system in a lasting manner.
In conjunction with the present invention, the utilization of the leakage air to ensure the parking state has been described; it has been assumed in the preceding text that the leakage, via which the leakage air flows out, is an undesirable property of the control valve device. However, it can even be provided that the control valve device is equipped with a leak in a targeted manner, in order thus to make a defined contribution to the force which ensures the parking state.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.